During the manufacture and forming of products from sheets or webs of plastic material, ovens are used to preheat the web of material prior to molding the plastic material into groups of plastic thin-walled articles. Typically, the oven forms part of a thermo-forming machine having a thermo-forming press. A number of different articles can be formed from a sheet or web of plastic material as it is fed from a storage roll. Accordingly, a thermo-forming press produces a large quantity of molded articles by intermittently passing a progressing succession of adjacent sections, or shot lengths of web into the press, after which the web is stamped or formed. The mold is also used to hold or clamp the web in a stationary location while it is being stamped. Hence, the web is fed into the thermo-forming press in an intermittent fashion, and in footprints defined by the size and shape of the mold (top and bottom die) of the press as it operates in cycles on the web.
A typical prior oven construction consists of an elongate oven having open leading and trailing ends. Heating elements inside the oven, for example resistance heaters, operate to heat successive portions of a sheet as it is intermittently delivered into the thermal forming press at a desired molding temperature. However, the ability to properly heat a continuous thermo-formable plastic sheet or web of material to a desired temperature depends upon the amount of energy transferred to the sheet, which is, in part, dependent upon the amount of time that the web passes through the oven. Typical oven constructions utilize an oven body having a finite length. Therefore, in order to tailor heat delivery to a thermo-formable plastic sheet prior to feeding the sheet into a thermo-forming press requires an adjustment of the stationary time that the wet, sits within the press and oven.
One problem encountered when attempting to adjust the heat delivery to a thermal forming sheet passing through a finite length oven is the necessity of tailoring the oven length to the particular application, or adjusting the stop time during which the thermo-forming press has locked the web in a stationary position. In normal operations, it is desirable to increase production rate. Therefore, it is desirable to operate a thermo-forming press in as short a cycle time as is physically possible by the constraints of the press operation and web deformation between pairs of interlocking dies. Therefore, it is desirable to tailor heat delivery to the web through some other means. One possibility is to controllably adjust the heat output from each of the thermal resistance elements carried within art oven. However, such an attempt at heat delivery requires careful monitoring of heat being delivered and time during activation for each of the elements. Hence, a complex control scheme is needed to track and target heat and delivery values for each of the elements in an oven.
Therefore, a need has arisen to provide for a simple and economical oven construction that enables the adjustable tailoring of heat delivery to a web of thermo-formable plastic sheet material to be passed through a thermo-forming press that operates on a staged or intermittent stamping principle.
The objective of the present invention is to provide an improved heat tunnel construction for varying shot lengths and heat delivery to a web of material that can always be tailored to relate to the different regions of the product being formed from the web.